In manufacturing a surface-mount printed circuit board, a stencil printer can be used to print solder paste onto the circuit board. Typically, a circuit board having a pattern of pads or some other conductive surface onto which solder paste will be deposited is automatically fed into the stencil printer; and one or more small holes or marks (known as “fiducials”) on the circuit board are used to properly align the circuit board with the stencil or screen of the stencil printer prior to printing solder paste onto the circuit board. In some systems, an optical alignment system is used to align the circuit board with the stencil. Such a stencil printer is described and depicted in U.S. Pat. No. RE 34,615, entitled, “Video Probe Aligning of Object to be Acted Upon,” which is incorporated herein by reference in its entirety.
Once the circuit board has been properly aligned with the stencil in the printer, the circuit board is raised to the stencil, solder paste is dispensed onto the stencil, and a wiper blade (or squeegee) traverses the stencil to force the solder paste through apertures in the stencil and onto the board. As the squeegee is moved across the stencil, the solder paste tends to roll in front of the blade, which desirably causes mixing and shearing of the solder paste so as to attain a desired viscosity to facilitate filling of the apertures in the screen or stencil. The solder paste typically is dispensed onto the stencil from a standard cartridge such as that manufactured by Systems Engineering and Management Co. (SEMCO), Vista, Calif. USA.
In some stencil printers, any excess solder paste remaining under the squeegee, after it has fully traversed the stencil, remains on the stencil when the squeegee is returned to its initial position for printing on a second circuit board. In some screen printers, a second squeegee moves across the stencil in the direction opposite to that of the first squeegee. The first squeegee and the second squeegee are used on alternating boards to continually pass the roll of solder paste over the apertures of a stencil to print on each successive circuit board. In the stencil printers that utilize two squeegees, there is still the problem that at the end of a manufacturing day, or when the stencil is to be changed, excess solder paste typically remains on the stencil and must be manually removed. Also, in these known printers, it is difficult to maintain a desirable viscosity because volatile solvents escape from the solder paste, thereby affecting the viscosity of the solder paste.
In these stencil printers, the squeegee blades are typically at a predetermined angle with respect to the stencil to apply downward pressure on the solder paste to force the solder paste through the apertures in the stencil as the squeegee is moved across the stencil. The angle of the blade is selected based on the speed at which the blade traverses the stencil and based on the desired downward pressure on the solder paste from the blade. It is desirable to maintain a consistent pressure on the solder paste as the squeegee traverses the stencil; however, in a typical printer, the pressure varies due to variations in paste viscosity throughout a production run and due to variations in the angle of the squeegee caused by deformation of the squeegee due to the pressure applied by the squeegee driving device.
Responding to some of the problems, noted above, improved solder-paste print heads are described in U.S. Pat. Nos. 5,947,022, 6,324,973 and 6,453,810, each of which are incorporated herein by reference in their entirety. These patents describe a movable print head having a cylindrical chamber including ports to which removable cartridges that supply solder paste are coupled. Solder paste is passed from the removable cartridges, into the cylindrical chamber, then out of a dispensing slot, through a stencil and onto a circuit board in a desired pattern.